*Proposers to the observing call for NEID should use this calculator to estimate
the exposure times for their observations given a target SNR or RV precision,
and the inverse of those functions. This exposure time calculator was written
by the NEID Science Team and is based on initial best-estimates for the total
system throughput as a function of wavelength, precision from the default data
pipeline, and based on template stellar spectra at vsini = 2 km/s and solar
metallicity. All throughput estimates are deliberately conservative, and there
is an additional 30% margin built-in on top of this conservatism.
This calculator will be updated as better estimates of system performance
become available, and as on-sky performance is measured.*

** About the NEID Exposure Time Calculator **

This calculator is based on precomputed grids with parameter bounds of:

2700 K < T_{eff} < 6600 K

3 < V-mag < 17

10 s < exptime < 3600 s

0.3" < seeing < 1.9" s

Interpolating beyond these bounds will result in errors in the returned RV values.

All calculations are based on BT-Settl synthetic stellar spectra (with logg = 4.5 and solar metallicity), available at: http://phoenix.astro.physik.uni-goettingen.de/?page_id=15

The grids use a NEID throughput model with R ~ 95,000 and 380 nm - 930 nm spectral coverage and a median KPNO atmospheric transmission curve. The calculator assumes airmass 1.1. For RV precision estimates, we exclude regions of the spectrum with 1% or deeper telluric features (based on TelFit model for WIYN observatory). Median WIYN seeing is 0.8".

**SNR is reported per 1D extracted pixel.** The NEID resolution element is approximately
5 pixels wide.

** Note that this exposure time calculator reports only the photon noise contribution to the RV precision. This contribution must be added in quadrature to the 27 cm/s instrumental floor to determine the total measurement precision. **

A python implementation of the NEID Exposure Time Calculator is available here. (https://bitbucket.org/erik_timmermann_noao/neid-etc)